This invention relates generally to Fault Current Controllers (FCCs) and Current Controllers (CCs) which are used in high power applications to limit excessive current in electrical circuits, such as electrical power generators, under sudden fault conditions, such as a lightning strike and other fault occurrences to limit damage to associated electrical distribution equipment. In particular, the present invention relates to Fault Current Controllers and Current Controllers employing a superconducting shield core reactor (SSCR) as a fault detection and control element in the current controller circuit.
Superconducting Fault Current Controllers (FCC) and Current Controller (CC) are of significant interest to electric utility companies desirous of reducing or eliminating damage due to excessive current conditions. An FCC is a variable impedance device, which can provide impedance at different levels in an electrical circuit which is operable under fault conditions. A CC is a variable impedance device in an electrical circuit under continuous (normal) operating conditions. Under continuous (normal) operation, a superconducting FCC or a superconducting CC insert very little impedance in the power circuit.
A distinction exists between a fault current limiter (FCL) and an FCC. An FCL is also a variable impedance device. However, an FCL can only insert a fixed (pre-determined) impedance in the circuit under fault conditions. An FCC, on the other hand, can adjust the fault current level (i.e. magnitude) by changing the impedance to be inserted into the circuit when a fault condition is sensed. Thus, an FCC is more versatile than an FCL. An FCC not only can limit the fault current, but also can control the fault current level by adjusting the impedance to be inserted in the electrical circuit to a pre-determined amount. There is also a distinction between an FCL and a CC. An FCL is a variable impedance device under fault conditions while a CC is a variable impedance device under normal operation. A Current Controller is also more flexible and versatile than an FCL because the former can be used as an FCL under fault conditions and as a Current Controller under normal operation. One feature common to FCC, CC, and FCL systems is that all these systems insert insignificant impedance in the primary electrical circuit under operation.
A wire wound into a coil with overlapping layers (turns) insulated from one another functions as an inductive element and is commonly used in a current limiting application. Winding the coil around a material having little resistance to the flow of magnetic flux, i.e., a material which is easily magnetized, increases the inductance. Electrically conductive coils are frequently wound around a ferromagnetic core to increase inductance. Inductance can be even further increased by using a xe2x80x9cclosed loop core,xe2x80x9d which is a core forming a ring or square or similar unbroken path with no air gaps. Alternatively, if a low inductance is desired, two coils may be wound in magnetic opposition on the same closed loop core, with the magnetic field of each coil canceling the other. This procures a low impedance effect. When there is an imbalance in the currents between the two coils, the impedance increases. The capability to alter the impedance of the inductor by controlling the balance of the magnetic flux density forms the basis for use of the coil as a fault current limiter.
One approach to fault current limiting using a pair of magnetically coupled coils is disclosed in xe2x80x9cRecovery Time of Superconducting Non-Inductive Reactor Type Fault Current Limiter,xe2x80x9d by T. Hoshino et al., Transactions on Magnetics, Volume. 32, No. 4, July 1996, which discloses the use of two superconducting coils with different crucial currents non-inductively wound on a magnetic core in magnetic opposition. Under normal operating conditions, both coils are in the superconducting state and there is little resistance across the two coils. Current is shared equally between the two coils and there is no inductive voltage drop either across the coils.
Under fault conditions one or both critical currents are exceeded to cause an imbalance in the currents in the coils and an increase in impedance for limiting the fault current. Because one of the coils must first become non-superconducting to provide the necessary resistance, restoration of normal operating conditions with removal of the fault may be delayed until the resistance in the coils decays to a low value and excessive heating may occur. Another approach to a superconducting fault current limiter is disclosed in xe2x80x9cTests of 100 kw High-Tc Superconducting Fault Current Limiter,xe2x80x9d by W. Paul et al., IEEE Transactions on Applied Superconductivity, Volume. 5, No. 2, June 1995, which discloses an inductive superconductor fault current limiter where a superconductor shield prevents the formation of a field in the ferromagnetic core. Because this device is triggered magnetically and carries the total current load in the circuit, high currents in the circuit under normal conditions restrict the number of turns in the windings and limit performance under fault conditions. The current limiting performance of inductive fault current limiters based on Bi-2212 high temperature superconducting tubes is discussed in xe2x80x9cShort Circuit Test Performance of Inductive High Tc Superconducting Fault Current Limiters,xe2x80x9d by D. W. A. Willen et al., IEEE Transactions on Applied Super-conductivity, Volume. 5, No. 2, June 1995.
A fault current limiter employing a superconductor shield core reactor (SSCR) is disclosed in my U.S. Pat. No. 5,892,644, issued Apr. 6, 1999 for xe2x80x9cPASSIVE FAULT CURRENT LIMITING DEVICExe2x80x9d.
The present invention is directed to a passive circuit employing a current detection or sensing device such as a superconductor shielded core reactor having a non-linear impedance such that it exhibits a low impedance under normal operating conditions and a high impedance under fault current conditions or other high current conditions above a predetermined normal value or range of values.
In one embodiment, the present invention includes a variable impedance in a secondary circuit magnetically coupled to the primary coil for inserting or xe2x80x9creflectingxe2x80x9d a variable impedance in the primary circuit under fault conditions, and thus controlling the current in the power circuit.
The present invention, in another embodiment, generates a variable current in a secondary circuit, not necessarily under fault conditions, but rather under normal operating conditions, thus allowing the utility operators to limit or control the current in the primary circuit under a range of normal conditions, and producing a continuous and variable current limiting effect, not previously capable of implementation in the art because no suitable control systems were known in the art for the power and current ranges contemplated by the apparatus of the present invention.
In another embodiment, a variable impedance is included in one secondary circuit and a controlled current is presented in a second secondary circuit for combined fault current control and continuous current control (FCC/CC).